1. Field of the Invention
The present invention relates, in general, to the art of filtering of a video signal in order to eliminate cross-effects.
2. Description of the Related Art
As is commonly known, a composite video signal comprises color information and luminance information. When a video signal is received by, for instance, a color television tuner, the luminance signal (Y) and the chrominance signals (U, V) need to be separated. For instance, in a PAL video signal, the video signal has a bandwidth of approximately 5 MHz, wherein the carrier of the color signal is located at about 4.43 MHz. In order to obtain the luminance signal Y, a band suppression filter is needed which suppresses the color carrier. On the other hand, in order to obtain the color signals U, V, a band-pass filter is needed which passes the color carrier and suppresses all other frequencies.
In practice, a 100% separation between the luminance signal Y and the color signals U, V cannot be achieved. After decoding, the luminance signal Y will be affected to some extent by the color signal, while the color signals U, V will in some extent be affected by the luminance signal. These two effects are indicated as cross-luminance effect and cross-color effect, respectively, or generally as cross-effects.
These cross-effects are visible in the video image as displayed on a TV-screen. For instance, in high frequency black/white patterns (stripe patterns), rainbow effects can be seen. On the other hand, at an intersection between two adjacent image parts of different colors, a black/white block pattern can be seen. Both of these effects (rainbow effect and block pattern) are not stationary, but move due to the 25 Hz offset of the video signal.
In order to eliminate, or at least reduce, these cross-effects, it is already known to use a 2-D comb filter or a 3-D comb filter.
A 2-D comb filter (two-dimensional) is operative on image information in two directions, i.e., horizontal and vertical. A disadvantage of 2-D comb filters is that they allow diagonal artifacts.
A 3-D comb filter (three-dimensional) compares information in two spatial dimensions, i.e., horizontal and vertical, and also in a temporal dimension. In other words, a 3-D comb filter determines changes with time. A 3-D comb filter can eliminate the problem of the diagonal artifacts known from 2-D comb filters. However, in order to be able to determine changes with time, a 3-D comb filter needs three field memories, which are relatively expensive. Further, the 3-D comb filter action can only be performed with respect to stationary parts of the image; if the image comprises moving parts, the filter behaves as a 2-D filter. The switching from 3-D operation to 2-D operation whenever a part of the image starts to move, and vice versa, is undesired.